Automatic transmissions include meshing gearing elements, which can be lubricated and cooled with Automatic Transmission Fluid (ATF). Ideally, the temperature of the ATF should be maintained in a narrow range. If the ATF is too hot, it is not as effective as a lubricant and is less effective in removing heat from components. Additionally, chemical reactions may occur which change the friction properties of clutches and brakes. If the ATF is too cold, it has high viscosity, causing drag and making it difficult to force the ATF to some of the components that need lubrication.
Similarly, engines may use a liquid coolant to remove heat. The coolant may also be used to provide heat, such as too the passenger cabin via a heater core. Therefore, it is desirable to maintain the temperature of the coolant in a narrow range. Heat is typically extracted from the coolant by a liquid to air heat exchanger called a radiator.
The operating temperature range for engine coolant is typically less than the operating temperature range for transmission fluid. Therefore, heat from the ATF may be transferred to engine coolant using a liquid to liquid heat exchanger called a transmission oil cooler. However, the additional heat input into the coolant may limit the ability to control the coolant temperature during period of very aggressive vehicle operation. When a vehicle starts operating from an initial cold condition, the engine coolant typically warms up to normal operating temperature more quickly than the ATF. Therefore, during this warm-up phase, the transmission oil cooler may be used to transfer heat from the coolant to the ATF, reducing the time that the transmission must operate with cold ATF. Although this improves the performance of the transmission, it may degrade the performance of the cabin heating system.